Spectroscopy
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Spectroscopy is one of the research areas of the Laboratory of Optics and Spectroscopy. The research work is focused on the following topics:
- Photoacoustic detectors
- Interferometers
- Mathematical methods of spectral analysis
- High resolution spectroscopy
Photoacoustic detectors
Photoacoustic detecting is one of the most sensitive methods to measure very small concentrations of gases. In this method the gas is closed into the chamber which has a window for external IR radiation. When certain gas specific wavelengths of IR radiation are absorbed the temeprature of the gas increases and also the pressure inside the chambers. The increase of the pressure is directly proportional to the consentration of the gas, and it can be measured by a sensitive microphone.
In the traditional photoacoustic cell the microphone is made of very thin film whose movements are determined capacitively. If it is replaced by the cantilever made of silicon whose movements are measured by the interferometer, the sensitivity of the method can be increased significantly. In the laboratory various constructions of the cantilever based photoacoustic cells have been developed. With these detectors ppt level (part-per-trillion) concentrations of methane have been measured.
Interferometers
The Laboratory of Optics and Spectroscopy has developed many interferometres, which have been used as Fourier transform spectrometers (FTS). As his doctoral thesis, Prof. Kauppinen constructed an FT-spectrometer, which possessed the world record of resolution for more than two decades (1972-1993). The instrument was the first in Scandinavia. Afterwards Kauppinen modified the spectrometer, and constructed the first FT-spectrometer in the world with cube corner mirrors. Even today this type of construction is the most widely used in commercial FT-spectrometers.
Recently, we made a new new invention, the carousel interferometer. It is a compact low resolution equipment, based on rotational mirror motion. This solution makes the carousel interferometer stable in modulation and immune to mechanical distortion. We have found that the carousel interferometer is very suitable even for ultraviolet (UV) region spectroscopy. In cooperation with the Technical Research Center of Finland we have studied the possibilities of using the carousel interferometer in an FT-UV analyzer to detect heavy metals in industrial stack gases.
As a result of cooperation of our research group and Temet Instruments inc., Helsinki, Finland, we have developed the portable FT-IR gas analyzer GASMET. During one second GASMET measures simultaneously the concentrations of almost all possible constituents in a gas mixture with a detection limit of one ppm. The instrument has no real competitors in the international market. The carousel interferometer has now been installed in the GASMET in order to achieve even better performance.
Mathematical methods of spectral analysis
The Laboratory of Optics and Spectroscopy has developed mathematical methods for signal processing, and for calculation of spectra. The mathematical methods include derivation, smoothing, deconvolution (opposite to smoothing), background elimination etc. Our methods for increasing the spectral resolution have become world-famous. They are able to exceed the level of de-smoothing attained by simple deconvolution.
The method of Fourier Self-Deconvolution has been widely used for fifteen years, and it is a universally approved procedure for spectral resolution enhancement. It has become a concept introduced in textbooks, and it is a tool that is used in almost all the commercial FT-IR spectrometers. A new, more efficient method is LOMEP, which is based on linear prediction (US patent). We have also developed another linear prediction -based line narrowing algorithm, the gulf tuning method (see the figure). Given the line shape and a piece of spectrum, these methods are able to narrow all the lines in this piece by a desired factor k. For concentration analysis the group has developed the multicomponent analysis procedure used in the GASMET gas analyzer (US patent), manufactured by Temet Instruments Inc.
In computing the spectrum of a registered signal there are two sources of error: the spectral aliasing due to discrete sampling, and the spectral smoothing due to signal truncation. The aliasing is an inherent property of sampling: high frequencies give identical samples with certain lower frequencies. The only remedy is to use dense sampling. On the other hand, the smoothing problem can be helped by extrapolating the signal, i.e., by calculating new sample data after the measured ones. The information for this extrapolation indeed exists for line spectra, if spectral lines, which are located near to each other, can be assumed to have an identical functional shape. A suitable method for carrying out the extrapolation is linear prediction. As a necessary prerequisite, Fourier Self-Deconvolution is needed to make the signal linearly predictable and to prevent the extrapolation from dying out.
We have used linear prediction in developing strategies for narrowing spectral lines, i.e., for enhancing the spectral resolution, as described above. In addition to resolution enhancement, we have applied linear prediction to find out spectral line shape, to correct spectral background and interference, and to reconstruct missing pieces to a band-limited, oversampled signal using as small amount of computation as possible.
High resolution spectroscopy
In 1972, Prof. Kauppinen constructed a Fourier transform spectrometer, which possessed the world record of resolution for more than two decades. Numerous IR-spectra have been measured with the highest resolution in the world. These spectra have been analyzed and published mainly in cooperation with foreign spectroscopists (about 100 publications). We have, e.g., measured and analyzed standard spectra of simple molecules (H2O, CO2, OCS, N2O, etc.) for the standard book "Handbook of Infrared Standards with Spectral Maps and Transition Assignments between 3 and 2600 mm", edited by G. Guelachvili and K. Narahari Rao, Academic Press INC, 1986. Our section of the book consists of 120 pages.
Of the other works related to the standards, worth mentioning is the interferometric measuring system which was developed in collaboration with the Helsinki University of Technology. This instrument serves as the Finnish standard of length.
Contact person: Tom Kuusela